The present invention relates to the detection of an endpoint in the etching of a substrate.
In the processing of a substrate to fabricate electronic devices, such as electrical circuits and displays, etching processes are carried out to etch patterns in the substrate that correspond to layers or components of the electronic devices. For example, the patterns may comprise gates, vias, contact holes, or interconnect lines. Typically, a patterned mask of etch-resistant features comprising resist or hard-mask is materials is formed on the substrate, and exposed areas of the substrate between the etch-resistant features are etched to form the patterns.
During the etching process, an endpoint detection method is used to evaluate and control the progress of etching through the dielectric layer, such as to stop or change the etching at a predetermined trench etch depth. In interferometric endpoint detection methods, as illustrated in FIG. 1 (Prior Art), a light beam 76 is directed onto the substrate 10 and a reflected light beam 78 emerges from the substrate 10. Constructive and destructive interference of portions of the reflected light beam 78 over time modulate the light beam 78 to form interference fringes, such as intensity maxima and minima. The reflected light beam 78 is detected by a detector that generates an interference signal, which is monitored to determine an endpoint of the etching process. When the reflection signal exhibits fringes that arise from the interference between a primary reflection 50 from the surface of the substrate 10 and reflections from the first couple of layers, such as from mainly the second layer 22. The interference fringes are used to measure the etch rate, the etch depth, and determine whether an etching process endpoint has been reached.
However, a portion 76b of the light beam 76 that is incident on the exposed areas 61 between the mask features 62 is partially transmitted to deep layers 23 below the first and second layers 30, 22, such as to third 23, or fourth (not shown) layers. The portions of the light beam 51-53 reflected from these deep layers 23 undesirably interferes with the primary reflection 50, adding noisy, redundant interference fringes to the reflected light beam 78 that make the meaningful interferences fringes in the reflection signal more difficult to identify. Another light beam portion 76a that is incident on the substrate 10 is reflected from the mask features 62, such as from the surface 17 of the mask 15 or even the layers 30, 22, 23 below the mask material. These extraneous reflections 40-44 also interfere with the reflected light beam 78 and add to the total reflection signal, decreasing the effective signal-to-noise ratio and possibly causing the indication of false endpoints.
As semiconductor devices are processed to be finer in scale, it is desirable to detect endpoint with higher precision and increased accuracy. The reflections 52, 53, 40-44 from the mask features 62 and deep layers 23 effectively limit the precision and accuracy of endpoint detection by adding noise to the reflection signal. This noise is sometimes removed using filters such as bandpass filters, which increase the complexity of endpoint detection and often do not entirely remove the noise. In order to etch shallower trenches, and etch trenches to a more exact depth, it would be desirable to have an endpoint detector capable of a higher precision. In conventional interferometric endpoint detectors, the noise that is added to the reflection signal degrades the ability to detect endpoint. For example, some conventional interferometric endpoint detectors have a minimum detectable depth of about 320 nm of a trench being etched.
Thus, it is desirable to interferometrically detect the endpoint of a substrate processing step to a higher precision. It is also desirable to interferometrically detect endpoint with less susceptibility to false endpoints, and thus improved accuracy.